Manuel Vicente
Manuel Vicente is the former head of Angola's state-owned oil company Sonangol (Reuters)

Three top Angolan officials have admitted to holding hidden stakes in a west African oil venture under investigation by US authorities that is linked to the Goldman Sachs-backed explorers Cobalt International Energy (CIE), according to the Financial Times, sending CIE stocks tumbling.

The ex-head of the national oil company and two senior generals had shares in Nazaki Oil and Gáz, on of Cobalt's local partners in the deepwater-drilling venture off the Angolan coast, two of the three men told the FT.

Shares in CIE are down 10.39 percent on the New York Stock Exchange at 10:55am local time.

Manuel Vicente, former head of state-owned Sonangol and current minister of state for economic co-ordination, General Manuel Helder Vieira Dias Junior, also known as Kopelipa, head of the presidency's military bureau, and General Leopoldino Fragoso do Nascimento were all identified.

Vicente and Kopelipa denied that they had influenced the government's decision to award Cobalt oil rights, telling the FT that the men were "always respecting all Angolan legislation applicable to such activities, not having committed any crime of abuse of power and/or trafficking of influence to obtain illicit shareholder advantages".

"Should the continued operation of Cobalt in the Angolan petroleum sector be unviable as a result of any failure to comply with any American law, there will certainly be [Angolan] and/or foreign entities interested in substituting for it in the assets it owns in Angola," the pair added.

Cobalt insisted that it had "at all times complied fully with both US and Angolan laws".

US regulators at the Securities and Exchange Commission started probing Cobalt's Angolan operations in February amid allegations of links between government officials and the company's local partners.

Cobalt, in which Goldman Sachs is the majority shareholder with a 19 percent stake and a founding investor, said it is fully co-operating with the authorities.